Chapter 12 Vocabulary and Study Guide Volcanoes 1) acid rain Moisture with a PH below 5.6 that falls to Earth as rain or snow and can damage forests, harm organisms, and corrode structures. Sulfurous gases emitted by volcanoes can mix with water vapor and form acid rain 2) tephra Bits of rock or solidified lava dropped from the air during an explosive volcanic eruption. These range in size from volcanic ash to volcanic bombs and blocks. 3) pyroclastic flow A ground-hugging avalanche of hot ash, pumice, and rock fragments that rushes down the side of a volcano during an eruption. The flow travels down the slope of the volcano at speeds up to 150 miles per hour. The temperature inside the flow of hot gases and rock can reach 1500 degrees Fahrenheit. These "stone winds" traveling at hurricane speeds kill or destroy everything in their path. The flows usually follow the curvature of the land to the valleys below the mountain. Sometimes a pyroclastic surge will jump ridges and flow down nearby valleys spreading the destruction into new areas. 4) intrusive A type of igneous rock feature that generally contains large crystals and forms when magma ( not lava!) cools slowly beneath Earth s surface. Magma underground cools very slowly, over thousands to millions of years. As it cools, elements combine to form common silicate minerals, the building blocks of igneous rocks. These mineral crystals can grow quite large if space allows. The mineral crystals within this type of rock are large enough to see without a microscope. There are many different types of intrusive igneous rocks but granite is the most common type. Intrusive igneous rock bodies such as batholiths, dikes, and sills form when magma solidifies underground. 5) extrusive This describes fine-grained igneous rock that forms when magma cools quickly at or near the Earth s surface. When lava erupts onto the Earth's surface, it cools quickly. If the lava cools in less than a day or two, there is no time for elements to form minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface. 6) lava Molten rock that flows from volcanoes onto Earth s surface. After magma reaches the surface it's called lava. 7) magma Hot, melted rock material beneath Earth s surface. Magma originates in the mantle. There, high temperatures and pressure cause some rocks to melt and form magma. After magma reaches the surface it's called lava. Magma rises to the surface because magma is less dense than rock. The types of magma are determined by the chemical composition and the physical properties of the magma. Three general types are recognized: basaltic magma, andesitic magma, and granitic magma. Basaltic magma has a high temperature and due to its flowing nature is the type of magma most commonly thought of when talking about volcanoes. Andesite magma has a moderate temperature and is moderately explosive. Granitic magma has a low temperature and is highly explosive. The way a volcano erupts is largely determined by the viscosity, or gooeyness of the magma. Viscosity is the resistance to flow (opposite of fluidity) and depends primarily on the composition of the magma, and its temperature. Water has low viscosity (flows easily) while syrup and honey have greater viscosity. High viscosity lavas flow slowly and typically cover small areas. Low viscosity magmas flow more rapidly and form lava flows that cover thousands of square kilometers.
Low viscosity magmas allow gases to escape easily whereas gas pressures can build up in high viscosity magmas resulting in violent eruptions. Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). The silica content is another factor in determining how viscous magma is. Higher SiO 2 (silica) content magmas have higher viscosity than lower SiO 2 content magmas (viscosity increases with increasing SiO 2 concentration in the magma). Granitic 8) caldera A large, circular shaped opening formed after an eruption of a volcano when the top of the volcano collapses. A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a very large volcanic eruption. If enough magma is ejected, the emptied chamber is unable to support the weight of the rock above it and it collapses. Many times, caldera will fill with water and become mountaintop lakes. Most calderas are massive, crater-like depressions that can cover many tens of square miles. 9) cinder cone volcano A steep-sided, loosely packed volcano formed when tephra falls to the ground. These generally do not have a lava flow, but when they erupt they are explosive and throw lava high into the air. Paricutin, in Mexico, is a cinder cone volcano. Cinder cones are made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent. Cinder cone volcanoes are the simplest and the most common type of volcano. They are steep sided circular or oval cone shapes of basaltic fragments and are much smaller and much simpler than composite volcanoes. 10) composite volcano Composite volcanoes, also known as a stratovolcanos, are found mostly where Earth's plates come together and one plate sinks beneath the other. Composite volcanoes are steep-sided volcanoes composed of many alternating layers of volcanic rocks, lava, volcanic ash, tephra, and pumice. Mt. Rainier and Mount St. Helens are examples of this type of volcano. The eruptions of Krakatoa in 1883, Mount Saint Helens in 1980 and Mount Pinatubo in 1992 were examples of large volcanic eruptions by composite volcanoes. Often, the large-volume explosions rapidly drain the lava beneath these mountains and cause the top to collapse to form large depressions called calderas. Later, these depressions fill with water and form beautiful lakes, like Crater Lake in Oregon. 11) shield volcano Broad, gently sloping volcano formed by quiet eruptions of basaltic lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of a flat, rounded shape, with a profile much like that of a warrior's shield. Shield volcanoes are the least explosive. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of shield volcanoes including Kilauea and Mauna Loa on the island of Hawaii, two of the world's most active volcanoes. 12) volcanic mountain A volcanic mountain is formed when molten material and ash reaches Earth s surface through a weak crustal area and piles up into a cone-shaped structure. The term volcano is only the rupture on the crust of Earth which allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. It is not a volcanic mountain until enough material has piled up to make it a mountain.
13) volcano Opening in Earth s surface that erupts allowing sulfurous gases, ash, and lava to escape from a magma chamber below the surface. They can form at Earth s plate boundaries, where plates move apart or together, and at hot spots. Volcanoes often are found in areas on Earth where plates collide and at these convergent plate boundaries, volcanoes tend to erupt more violently than they do in other areas. A volcano often forms a mountain when layers of lava and volcanic ash erupt and build up. Sulfurous gases emitted by volcanoes can mix with water vapor and form acid rain. More than 600 of Earth s volcanoes are currently active but Kilauea in Hawaii is the world s most active volcano. The Pacific Ring of Fire is an area around the Pacific Ocean where over 75% of the volcanoes on Earth are found. The Ring of Fire outlines the borders of the Pacific Plate and other major tectonic plates and includes the western coast of the United States. 14) hot spot The result of an unusually hot area at the boundary between Earth s mantle and core that forms volcanoes when melted rock is forced upwards and breaks through the crust. Mantle plumes are these areas of hot, upwelling mantle. A hot spot develops above the mantle plume. Hotspots may be far from tectonic plate boundaries. A volcanic hotspot is where lava pushes up from under the mantle and creates a volcano. The earth's plates move along and another volcano is created later. This creates a chain of volcanoes, such as in Hawaii. The Hawaiian Islands are formed because of hot spots. The Hawaiian hot spot has been active at least 70 million years, producing a volcanic chain that extends 3,750 miles (6,000 km) across the northwest Pacific Ocean. 15) volcanic neck The solid igneous core of a volcano left behind after the softer portions of the cone have been eroded. A volcanic neck is the remnant of an old eroded volcano. Differences in resistance to erosion cause a volcanic neck to form. As the volcano died, the last bit of lava inside of the volcanoes opening, or neck, cooled and hardened. Over many hundreds of thousands of years the material around the neck is removed by erosion, leaving only the harder neck behind. Volcanic necks are sometimes called volcanic plugs. When present, a plug can cause an extreme build-up of pressure if the volcano becomes active again and magma is trapped beneath the plug. This can lead to a very explosive eruption. 16) Devil's Tower in Wyoming is an example of a volcanic neck and is seen in the picture above. Devils Tower was the first declared United States National Monument, established on September 24, 1906, by President Theodore Roosevelt. According to Native American legend, two girls went out to play and were spotted by several giant bears who began to chase them. In an effort to escape the bears, the girls climbed atop a rock, fell to their knees, and prayed to the Great Spirit to save them. Hearing their prayers, the Great Spirit made the rock rise from the ground towards the heavens so that the bears could not reach the girls. The bears, in an effort to climb the rock, left deep claw marks in the sides, which had become too steep to climb. 17) crater A steep-walled, usually circular, depression around a volcano s vent. This is where the lava, ash and rock erupt out of a volcano. In most situations, the volcano crater is located at the top of the volcano. Think of a classic cone-shaped volcano, with steep sides and a slightly flattened top. If you could climb up to the top of the volcano and peer over the edge, you would look down into the volcano crater. And when the volcano does erupt, the material comes out of this volcano crater. 18) vent An opening in Earth s crust where magma is force up and flows out on to Earth s surface as lava, forming a volcano. The opening in a volcano, even if it not at the peak of the volcanic mountain is a vent if that is where the pyroclastic material is coming from. There is usually a central vent but there may also be secondary vents that also allow material to be released from inside the Earth.
19) batholith The largest intrusive igneous rock body that forms when magma being forced upward toward Earth s crust cools slowly and solidifies underground. These magma bodies cool very slowly before they reach Earth's surface. Batholiths are usually composed of coarse-grained rocks like granite or quartz, have an irregular shape, with side walls that incline steeply, and may be 40 sq mi or more wide and 6 9 mi deep. 20) dike An intrusive igneous rock feature formed when magma is squeezed through a vertical crack that cuts across rock layers and hardens underground. Dikes are formed across vertical cracks, and sills are formed across horizontal ones. Dikes are usually visible because they are at a different angle, and usually have different color and texture than the rock surrounding them. Dikes are up and down. 21) sill An intrusive igneous rock feature formed when magma is squeezed into a horizontal crack between layers of rock and hardens underground. Dikes are formed across vertical cracks, and sills are formed across horizontal ones. Sills parallel beds (layers) and foliations in the surrounding rock. Sills are back and forth. 22) divergent boundary when two tectonic plates are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and form mid-ocean ridges. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. 23) convergent boundary when two tectonic plates are moving towards each other. Active volcanoes are most likely to form at convergent oceanic continental boundaries. When two oceanic plates converge, the denser one subducts, or sinks, beneath the other, causing volcanic islands and trenches to form. When two continental plates collide there is usually little or no subduction, so there is rarely volcanic activity but rather a lot of mountain building. oceanic-continental continental-continental oceanic-oceanic 24) Paricutin a large cinder cone volcano that grew in a corn field in Mexico in 1943. Parícutin is a cinder cone volcano in the Mexican State of Michoacán, close to a lava-covered village of the same name. The volcano is unique in the fact that its evolution from creation to extinction was witnessed, observed and studied. It appears on many versions of the Seven Natural Wonders of the World. The volcano grew quickly, reaching five stories tall in just a week. Paricutin was active for nine years, during which time it destroyed the nearby town San Juan and covered a large area of around a hundred square miles with ash. 25) Kilauea in Hawaii is the world s most active volcano. Kilauea is a shield volcano in the Hawaiian Islands, the most active of the five volcanoes that together form the island of Hawaii. Located along the southern shore of the island, the volcano, at 300,000 to 600,000 years old, is the second youngest product of the Hawaiian hotspot. Kilauea's current eruption dates back to January 3, 1983, and is by far its longest-lived historical period of activity, as well as one of the longest-lived eruptions in the world; as of January 2011, the eruption has produced 3.5 cubic km of lava and resurfaced 123.2 km 2 (48 sq mi) of land.
26) Krakatau A very violent eruption in 1883 that collapsed an entire island into a magma chamber. It was one of the biggest, and loudest, eruptions in recorded history and was primarily andesitic in composition. The eruption also unleashed huge tsunamis (killing more than 36,000 people) and destroying over two-thirds of the island. The explosion is considered to be the loudest sound ever heard in modern history, with reports of it being heard up to 3,000 miles (4,800 km) from its point of origin. The shock waves from the explosion were recorded around the globe. 27) aa lava flow one of the two distinct types of hardened lava you ll likely see in Hawaii. It is believed that the name a a came from the noises someone might make while walking over it in bare feet. Maybe an alternative name should be ouch ouch or ow ow. A'a lava looks completely different than pahoehoe lava as it is formed. Whereas pahoehoe lava flows smoothly like water or molasses, a'a lava tumbles in the form of small rocks with very jagged sharp edges. The rocks are porous and very jagged. The rocks are very lightweight, as opposed to pahoehoe that is extremely dense, and a'a rocks tend to pile up on each other with a lava front that might be a few feet to 40 or 50 feet high of tumbling red hot sharp rocks. It flows slowly and is intensely hot. Below the surface a'a is extremely dense. A'a is very difficult, if not nearly impossible, to walk on (at least without getting hurt). 28) pahoehoe lava flow one of the two distinct types of hardened lava you ll see in Hawaii. Pahoehoe lava comes out smooth and dense and can form large areas that resemble flat parking lots or smooth bumps. In general, pahoehoe is very easy to walk on. Pahoehoe lava is one of the most interesting forms of lava. Since it tends to flow more as a thick liquid it can pour uphill as well as downhill and can create a huge variety of interesting shapes. This type of lava flow is more fluid, develops a smooth skin, and forms ropelike patterns when it cools. Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere. This causes a surface skin to form, although it is still very hot and behaves in a plastic fashion, capable of deformation. Initially the surface skin is smooth, but often inflates with molten lava and expands to form pahoehoe toes or rolls to form ropey pahoehoe. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent. 29) Iceland Islands like Iceland are often formed due to rifts in the Earth's surface, plate movement, and cooling lava flows. Active volcanism is currently taking place along all oceanic ridges on Earth, but most of this volcanism is submarine, underwater, volcanism. One place where an oceanic ridge reaches above sea level is at Iceland, along the Mid-Atlantic Ridge. Here, most eruptions are basaltic in nature, but, many are very explosive. As seen in the map, the Mid-Atlantic ridge runs directly through Iceland. SiO 2 30) silica A compound composed of silicone and oxygen. Silica is most commonly found in nature as quartz as well as in various living organisms. A major portion of all magma is silica. The amount of silica, along with water vapor, that is trapped in magma determines the explosiveness of a volcanic eruption. The more silica there is in magma, the stiffer it is and the more it resists flow. 31) Soufriére Hills Volcano an example of a composite volcano formed at a subduction zone in Montserrat. Its existence is due to the subduction of the Atlantic under the Caribbean plate. Its first eruption started in 1995 and is still ongoing. Much of the area is now buried beneath a thick layer of ash and mud. It is one of the most studied volcanoes in the world today.
32) basaltic magma This type of magma is low silica, fluid, and produces a quiet, non-explosive eruption. High levels of silica cause magma to become more viscous. When rock in the upper mantle melts, basaltic magma usually forms. The low silica content of basaltic magma makes it low viscosity so that dissolved gasses can escape. The resulting volcanoes have quiet eruptions such as Kilauea. 33) granitic magma Granitic magma eruptions are the most explosive. High levels of silica cause magma to become thicker. Granitic magma is thick and stiff and contains lots of silica and dissolved gasses. Granitic magma has the most silica and has the highest viscosity. 34) andesitic magma Andesetic magma has an intermediate amount of silica which gives it an intermediate viscosity, in between basaltic and granitic magmas.